1. Field of the Invention
The present invention generally relates to modem communications, and in particular to a system and method for protecting the digital access arrangement of a modem when the modem is connected to a digital telephone line.
2. Related Art
Digital devices, such as computers, can utilize analog modems to communicate between each other across a central telephone office""s Public Switched Telephone Network (PSTN). PSTN telephone lines are analog telephone lines. The analog modem provides communication by modulating outgoing digital signals of the digital device to allow usability of the outgoing signal by the PSTN telephone lines. Similarly, the analog modem demodulates incoming analog telephone signals of the PSTN telephone line to allow usability of the incoming signal by the digital device.
Typically, analog modems contain delicate integrated circuit components and a digital access arrangement in order to provide communication across the PSTN telephone lines. The digital access arrangement (DAA) is a circuit that interfaces high voltage devices, such as telephone lines, with the modem""s delicate integrated circuit components, which usually operate at voltage levels ranging from only 3 to 5 volts. The DAA typically isolates the high voltage and provides a barrier from the telephone line voltages. Also, the DAA protects the modem from harmful high voltage spikes originating from the telephone lines. Other non-protective functions of the DAA include, detecting an incoming ring signal, controlling seizure and release of the telephone line, drawing current from the telephone line to maintain connection with the central telephone office, and passing the modem audio tones in both directions, with minimum distortion. It should be noted that the term audio tones used here is a brief way of describing the complex waveforms normally associated with the complex protocols utilized in modem communications.
Although a PSTN telephone line is typically biased with 48 volts (nominal), the PSTN voltage is extremely current-limited at the central office. Namely, in normal applications, the 48 volts drops to a nominal 5 to 15 volts when a compatible telephone desk set, or equivalent, is connected to the telephone line. The central office""s current limiting circuitry, together with the actual resistance of the telephone line is commonly referred to as line resistance. It should be noted that the main element of the line resistance is the current limiting circuitry. Because of this current limiting circuitry, compatible telephones, analog modems and related devices are not required to dissipate large amounts of energy. Thus, current DAA""s of analog modems sufficiently protect the modem from PSTN telephone lines.
However, with the advent of alternative telecommunication schemes, the delicate circuitry of analog modems are at risk. This is because there is no telephone line standard for non-PSTN systems, and many different protocols are in common use that are incompatible with PSTN telephone devices, such as analog modems. In some cases, the incompatibility can cause damage to the analog modems. Many common alternative schemes utilize radio frequency (RF) carriers on the telephone line, modulated by a digital stream of data. Also, many of these alternative schemes permit numerous telephones to share the same physical telephone line, with each individual telephone having an assigned time slot. One such system in use is the T1 system, which uses a carrier of 1.544 MHz. The telephone line used in these schemes is usually referred to as a digital line. These schemes are commonly found in office buildings, hotels and similar environments.
In addition, in these alternative systems it is customary to bias the telephone lines with approximately 48 volts. However, this biasing is utilized to power remote compatible telephones and to provide ongoing current, commonly referred to as sealing current. Sealing current prevents circuit deterioration due to corrosion and oxidation that plagues no-current (dry) circuitry. Thus, the bias voltage for these alternate systems is not current limited like the PSTN telephone lines. As such, these alternate systems can deliver more than 250 milliamps (12 watts) of power to devices connected to these alternate systems. Therefore, connecting a non-compatible device, such as an analog modem, can result in overheating and rapid destruction of the non-compatible device.
Unfortunately, these new alternative schemes are often undetected by the analog modem user. This is because it is impossible for a user to visibly determine whether a random telephone line jack connects to a PSTN telephone line or a digital line because both systems typically use the same RJ-11 connector jack. Consequently, an incompatible connection by an unwary modem user can cause damage to the delicate components of the modem. Many current analog modems are not designed to protect the DAA and delicate integrated circuits from potentially hazardous digital telephone lines. This is because most analog modems are designed with the assumption that the user is knowingly connecting the analog modem to a PSTN telephone line only.
One current attempt to solve this problem involves adding a small precision resistor in series with the telephone line and measuring the voltage drop across this resistor. However, the added resistance drops the available DC voltage, which in turn degrades performance at low currents. Also, since this technique requires tightly specified parts that are expensive, the overall cost of the modem is unduly increased. Another technique involves using a handheld detection device. The handheld detection device determines whether a telephone line is a digital or a PSTN telephone line when the device is inserted into the telephone line jack in question. However, this technique is undesirable because it requires the user to purchase a separate product, requires inconvenient insertion of the device into a telephone jack before each use of the modem, and is subject to human error, i.e., forgetfulness and improper use.
Therefore what is needed is a digital line protection system and method that automatically protects the analog modem from potentially hazardous digital telephone lines without adding undue expense and user inconvenience. Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a system and method for protecting an electronic device connected to a telephone line.
The system includes a detector, alarm circuit and protector. The detector tests the voltage drop shortly after the modem seizes the telephone line and determines the overall resistance of the telephone line and the central office limiting circuitry. As previously discussed, a PSTN telephone line will cause a major voltage drop, while a digital line will not cause any significant drop at all. The system uses this logic to determine if the line is a digital line after testing the voltage drop. The alarm circuit recycles the ring signal of the telephone line and utilizes the ring detect signal as an alarm signal if the line is found to be a digital line. The alarm circuit can, if necessary, activate a current limiting protective circuit, drop the telephone connection, or activate other suitable functions as desired. The protector is a specialized circuit to protect delicate capacitors of certain circuits of the modem, such as the audio-transformer circuit, from damage during brief sequences, such as digital line test sequences. The protector can be a surge current protector for diverting potentially damaging energy produced by the digital line from sensitive components of the electronic device during connection with the digital line.
The system and method of the present invention does not degrade the performance of the electronic device, such as a modem, and does not require tightly specified or expensive parts. Also, the system and method of the present invention is convenient because it automatically protects the modem from potentially damaging digital telephone lines and eliminates user inconvenience without adding undue expense.
The foregoing and still further features and advantages of the present invention as well as a more complete understanding thereof will be made apparent from a study of the following detailed description of the invention in connection with the accompanying drawings and appended claims.